Method for CVD process control for enhancing device performance

ABSTRACT

A method and system for controlling the introduction of a species according to a determined concentration profile of a film comprising the species introduced on a substrate. In one aspect, the method comprises controlling the flow rate of a species according to a determined concentration profile of a film introduced on a substrate, and introducing a film on a substrate, the film comprising the species at a first concentration at a first point in the film and a second concentration different than the first concentration at a second point in the film. Also, a bipolar transistor including a collector layer of a first conductivity type, a base layer of a second conductivity type forming a first junction with the collector layer, and an emitter layer of the first conductivity type forming a second junction with the base layer. An electrode configured to direct carriers through the emitter layer to the base layer and into the collector layer is also included. In one embodiment, at least one of the first junction and the second junction is between different semiconductor materials to form at least one heterojunction. The heterojunction has a concentration profile of a semiconductor material such that an electric field changes in an opposite way to that of a mobility change.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to semiconductor processing techniques, moreparticularly, to controlling constituents of a film introduced onto asubstrate.

2. Description of Related Art

In the formation of modern integrated circuit devices, many constituentsare introduced to a substrate such as a wafer to form films. Typicalfilms include dielectric material films, such as transistor gate oxideor interconnect isolation films, as well as conductive material orsemiconducting material films. Interconnect metal films and polysiliconelectrode films, respectively, are examples of conductive andsemiconducting material films.

In addition to the above-noted material films, other constituents areoften introduced onto a substrate such as a wafer or a structure on asubstrate to change the chemical or conductive properties of thesubstrate or the structure. Examples of this type of constituentintroduction includes, for example, the deposition of a refractory metalonto an electrode or junction to form a silicide and the deposition ofgermane onto a substrate to form a silicon germanium junction in abipolar transistor. The introduction of constituents onto a substrate orstructure on a substrate such as described is referred to herein as asubset of film formation.

One way to enhance the performance of integrated circuit devices is toimprove control of the introduction of the constituents, such asimproved control of the introduction of process gas species indeposition introduction. Many wafer process chambers, including the EPICentura system, commercially available from Applied Materials, Inc. ofSunnyvale, Calif., utilize mass flow controllers to introduce processgas species. In general, a mass flow controller functions by permittinga desired flow rate of a gas species based on an input signal to themass flow controller demanding the flow rate. The concentration profileof a species constituent within a film deposited on a substrate is thena function of the mass flow rate of species introduced. In general, therelationship between a species concentration profile or gradientintroduced into or onto a substrate, for example a wafer, and the massflow rate of the species introduced is not necessarily linear.

In general, mass flow controllers are used to either supply a constantflow rate or a variable flow rate from a first flow set point to asecond flow set point over a period of time. One common flow rampbetween a first set point and a second set point is a linear flow ramp.A linear ramp, however, does not necessarily produce a desiredconcentration profile, e.g., a linear profile, of the species in theintroduced film. In the example of a species of germane (GeH₄)introduced to form a silicon-germanium film, a graded film is desirablein many situations. The desired graded profile in the film, for examplegermanium concentration profile, may be linear or non-linear. The methodto control a mass flow controller to precisely control the amount offlow and produce the desired germanium concentration profile in thejunction, whether it is linear or non-linear, is of significantimportance. In commercial use, targeting a desired concentrationprofile, for example a linear profile, has generally not proved possiblethrough a linearly increasing or decreasing constant flow introductionof the germanium species by a mass flow controller.

What is needed is a way to control the introduction of a species to forma film having a desired concentration profile of the species in thefilm. The ability to quantitatively control the introduction of aspecies through a mass flow controller to form a film with a specificfilm thickness is also desirable.

SUMMARY OF THE INVENTION

A method and system for controlling the introduction of a species of afilm comprising the species introduced on a substrate is disclosed. Inone aspect, the method comprises controlling the flow rate of a speciesaccording to a determined graded concentration profile of a filmintroduced on a substrate, and introducing a film on a substrate, thefilm comprising the species at a first concentration at a first point inthe growth of the film and a second concentration different than thefirst concentration at a second point in the growth of the film. In oneembodiment, the concentration profile used to control the flow rate isestablished by experimentally determining a concentration of the speciesintroduced on a substrate for a first plurality of flow rates anddetermining an introduction rate, e.g., a growth rate, of the speciesintroduced, e.g., grown on a substrate. According to the invention, moreaccurate control of a species concentration in a formed film can beobtained over prior art methods. The invention also offers the abilityto control the amount or the thickness of a film formed on a substrate.

A bipolar transistor is also disclosed. In one embodiment, the bipolartransistor includes a collector layer of a first conductivity type, abase layer of a second conductivity type forming a first junction withthe collector layer, and an emitter layer of the first conductivity typeforming a second junction with the base layer. An electrode configuredto direct carriers through the emitter layer to the base layer and intothe collector layer is also included. In this embodiment, at least oneof the first junction and the second junction is between differentsemiconductor materials to form at least one heterojunction. Theheterojunction has a concentration profile of a semiconductor materialsuch that an electric field changes in an opposite way to that of amobility change.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a side view of a portion of a substratehaving a film with a graded concentration profile introduced accordingto an embodiment of the invention.

FIG. 2 illustrates the concentration gradient of a germanium species ina film according to an embodiment of the invention.

FIG. 3 illustrates a schematic view of an embodiment of a system forintroducing a species to a substrate according to the invention.

FIG. 4 illustrates a curve fit of the experimentally-determinedconcentration of germanium of a film introduced according to sixdiscrete germane flow rates and a constant silane flow rate.

FIG. 5 illustrates the experimentally-determined growth rate of silicongermanium in a film introduced on a substrate for six discrete germaneflow rates and a constant silane flow rate.

FIG. 6 illustrates a block diagram for the introduction of a germaniumspecies to a substrate in accordance with an embodiment of theinvention.

FIG. 7 is a Secondary Ion Mass Spectroscopy (SIMS) profile of anepitaxial silicon-germanium film introduced according to the inventionto have a linearly graded profile of germanium.

FIG. 8 is the flow rate of germane (GeH₄) per unit time to produce thegraded profile of FIG. 7.

FIG. 9 is a SIMS profile of an epitaxial silicon-germanium filmintroduced according to the invention to have a concave graded profileof germanium.

FIG. 10 is the flow rate of GeH₄ per unit time to produce the gradedprofile of FIG. 9.

FIG. 11 schematically illustrates a heterojunction bipolar transistorformed according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A method and a system for the controlled introduction of a species to asubstrate are disclosed. In one embodiment, the method includescontrolling the flow rate of a species into a chamber according todetermined concentration and introduction rate profiles to introduce(e.g., deposit) a film on a substrate in the chamber. The determinedconcentration and introduction rate profiles may be established throughexperimental data related to a concentration of a species in a formedfilm according to a plurality of selected flow rates of the speciesconstituent (hereinafter “species”) into the chamber. This informationis utilized to adjust the introduction rate of a species per unit timeto form a film having a desired concentration profile as well as adesired thickness.

FIG. 1 shows a side view of a portion of a semiconductor substratehaving a silicon germanium (Si_(1−x)Ge_(x)) epitaxially-introduced filmthereon. Structure 10 includes substrate 25 that is, for example, asilicon semiconductor wafer with Si_(1−x)Ge_(x) film 30 introduced on asurface thereof. Si_(1−x)Ge_(x) film 30, in this embodiment, has agraded concentration profile of germanium (Ge), represented byconcentration points 35 and 40. Concentration points 35 and 40represent, for example, two of many concentration points. In oneexample, the concentration profile from concentration point 35 toconcentration point 40 is desired to be linear with the highestconcentration of Ge present at concentration point 40 and the lowestconcentration at concentration point 35. In one example, theconcentration profile of Ge in Si_(1−x)Ge_(x) film 30 varies linearlyfrom a concentration of approximately zero percent Ge at concentrationpoint 35 to a concentration of 20 percent Ge at concentration point 40.

FIG. 2 graphically represents the concentration profile of germanium inSi_(1−x)Ge_(x) film 30 of FIG. 1. In this representation, theconcentration profile is measured from the surface of the film(represented by concentration point 35) to the silicon-Si_(1−x)Ge_(x)film interface (represented by concentration point 40). Thus, the filmthickness is measured from the surface of film 30 to the interface offilm 30 and substrate 25. In one example, denoted by the solid line, theconcentration profile varies in a linear fashion through the film. It isto be appreciated that the invention method and system is capable ofproducing a variety of concentration profiles, including non-linearprofiles such as profile 210 and profile 220 in FIG. 2.

FIG. 3 is an example of a process environment utilizing a system of theinvention to introduce a species to form a film such as film 30 onsubstrate 25 of FIG. 1. In this embodiment, an EPI Centura system,commercially available from Applied Materials, Inc. of Sunnyvale,Calif., modified according to the invention is described. It is to beappreciated that the system is not limited to an EPI Centura system butcan be accommodated in other systems, particularly where a mass flowcontroller is utilized to introduce a species into a reaction chamber. ASi_(1−x)Ge_(x) chemical vapor deposition (CVD) film formation process isalso described. Similarly, it is to be appreciated that the invention isnot limited to CVD Si_(1−x)Ge_(x) film formation systems but will applyto other systems and methods, particularly where a species is introducedinto a chamber to form a film.

Referring to FIG. 3, the system includes chamber 100 that accommodatessubstrate 25, such as a semiconductor wafer, for processing. Substrate25 is seated on stage 125 that is, in one embodiment, a susceptor plate.Heating lamps 102 and 104 are used to heat substrate 25. Processor 110controls the temperature and pressure inside chamber 100. Thetemperature is measured via, for example, pyrometers 106 and 108 coupledto the chamber. Similarly, the pressure may be monitored by one or morepressure sensors, such as BARATRON® pressure sensors, commerciallyavailable from MKS Instruments of Andover, Mass. and regulated by apressure control value. In the schematic illustration shown in FIG. 3,pyrometer 106 and pyrometer 108 are coupled to processor 110 throughsignal line 115. Processor 110 uses received information about thesubstrate temperature to control heat lamps 102 and 104. The one or morepressure sensors are coupled to processor 110 through signal line 135.Processor 110 uses received information about the chamber pressure tocontrol the pressure through, for example, controlling a vacuum sourceand a pressure control value coupled to the chamber.

Processor 110 also controls the entry of constituents into chamber 100.In one embodiment, the system includes at least two source gases 120 and130 coupled to manifold 105. Processor 110 controls the introduction ofeach of source gas 120 and source gas 130, as desired, through manifold105 and controls the flow of the source gas or gases through mass flowcontrollers 160 and 162, respectively. For use in a Si_(1−x)Ge_(x) filmformation process, mass flow controller 160 is, for example, a onestandard liter per minute (SLM) of silane (SiH₄) unit and mass flowcontroller 162 is a 150 standard cubic centimeters per minute (sccm) ofgermane (GeH₄) unit. Processor 110 also controls the introduction of aprocess gas (source gas 180), such as for example, nitrogen (N₂) orhydrogen (H₂), through mass flow controller 168 as known in the art.Each mass flow controller is, for example, a unit commercially availablefrom UNIT Instruments, Inc. of Yorba Linda, Calif.

In one embodiment, processor 110 controls the introduction of a sourcegas to form a Si_(1−x)Ge_(x) film on substrate 25, such asSi_(1−x)Ge_(x) film 30 in FIG. 1. Source gas 120 is, for example, theconstituent silane (SiH₄) and source gas 130 is, for example, theconstituent germane (GeH₄). In this embodiment, one goal is to introducea film having a concentration gradient of the species germanium (Ge)through the thickness of the Si_(1−x)Ge_(x) film. Still further, thisgradient is desired, in one embodiment, to be linear between aconcentration of Ge at a surface of the film (concentration point 35 ofFIG. 1) of zero percent and a maximum at an interface between thesilicon wafer and the film (concentration point 40 of FIG. 1).

In general, mass flow controllers, such as mass flow controller 160 andmass flow controller 162, can vary (e.g., increase or decrease) the flowrate change of a species introduced into a chamber. The concentrationchange of a species such as Ge over a thickness of a film may beaccomplished at the mass flow controller by changing the flow rate ofthe source gas into chamber 100. For any measurable control, this flowrate change is generally linear. However, a linear flow rate change, forexample, from higher to lower mass flow and thus lower introductionamount of species, does not necessarily produce a linear concentrationgradient of the species in the formed film. This is particularly thecase with the constituent GeH₄, where a linear increase or decrease offlow rate does not generally result in a linear change in concentrationof the species constituent Ge in the formed film. Instead, aconcentration profile of Ge in a formed film generally more closelyresembles the convex profile represented by dashed line 210. Accordingto the invention, however, a concentration profile such as representedby line 200 may be obtained by controlling mass flow controller 162 tointroduce source gas 130 at a non-linear rate.

In an embodiment of the invention, a method is presented wherein adesired concentration profile of the species, including the linearconcentration profile illustrated in FIG. 2 (line 200), is produced.According to this method, experimental determinations of theconcentration of a species such as Ge is measured in a film formedaccording to a plurality of flow rates of the constituent GeH₄ throughmass flow controller 162 on a sacrificial wafer. In one embodiment, theconcentration of Ge in a film formed on a wafer is measured for sixdiscrete flow rates of GeH₄ through mass flow controller 162. Eachexperimental measurement corresponds to a single unit of film introducedon a wafer by introducing a constant flow rate of GeH₄ through mass flowcontroller 162. In one embodiment, six concentrations of Ge in sixdiscrete films introduced by six discrete GeH₄ flow rates through massflow controller 162 on six wafers are measured. Each film is analyzedfor species Ge concentration through analytical methods such asSecondary Ion Mass Spectroscopy (SIMS), x-ray diffraction, orellipsometry. The six discrete flow rates are plotted versus the Geconcentration in a corresponding film as illustrated in FIG. 4. In thisexample, a single experimental measurement is obtained from a film on asingle wafer by placing a sacrificial wafer in chamber 100 and thereaction conditions of the chamber established. In one embodiment, afilm is formed at a chamber pressure of 100 Torr and a temperature of680° C. In one example, process gas is introduced in chamber 100according to the following flow rate recipe:

Source Gas Constituent Flow Rate 120 SiH₄ 1 SLM 130 GeH₄ varied 180 H₂30 SLM

It is to be appreciated, that other recipes may be utilized to introducethe films on the wafers. Such recipes will generally depend on thedesired process parameters. For example, in the introduction of aSi_(1−x)Ge_(x) film, additional constituents such as hydrochloric acid(HCl), may be added to modify the properties of the film. One objectivein collecting the experimental data is to mimic the desired processconditions as closely as possible.

According to the above recipe, six flow rates of source gas 130 of theconstituent GeH₄ are selected between 0 and 300 sccm. It is to beappreciated that GeH₄ flow rates higher than 300 sccm can be selected.One limit of GeH₄ flow may be considered as one beyond which the Geconcentration in the introduced film will not further increase for anincrease in the GeH₄ flow rate. A corresponding concentration of Ge ismeasured in a film formed on the sacrificial wafer. Once the data iscollected, a curve is established through a curve fit algorithm such asa Gauss-Jurdan algorithm. FIG. 4 illustrates the curve fit for sixpoints. In one example, a Gauss-Jurdan numerical algorithm is used tocalculate the coefficients of a third order polynomial that best fitsthe six experimental measurements. This method of curve fitting is knownas the Least Square Fit (LSF) method of curve fit. It is to beappreciated that the Gauss-Jurdan numerical algorithm is not the onlymethod to calculate coefficients of an LSF polynomial. Similarly, theLSF method as well as the Gauss-Jurdan method are not limited to sixdata points but may be used, for example, with as few as three datapoints or more than six data points.

According to an embodiment of the invention, the flow rate of GeH₄(e.g., six discrete flow rate measurements) is also measured against theintroduction rate, e.g., growth rate, of the Si_(1−x)Ge_(x) filmintroduced on the sacrificial wafer. It is to be appreciated that thesame six flow rates as utilized in FIG. 4 may be utilized to compare thegrowth rate of Si_(1−x)Ge_(x). In one embodiment, theexperimentally-obtained Si_(1−x)G_(e) growth rates are measured from thesame film grown on the same six sacrificial wafers used to measure Geconcentration. FIG. 5 shows a plot of GeH₄ flow rate versusSi_(x)Ge_(1−x) growth rate and a curve fit through the plotted points.The same numerical method of LSF is used to determine the best curve fitto the experimentally-obtained growth rate measurements. Thecoefficients of the third order polynomial are calculated using, forexample, the Gauss-Jurdan method noted above with respect to FIG. 4.

The experimentally-determined data for concentration of a species as afunction of flow rate and the experimentally-determined data for growthrate as a function of flow rate is input into processor 110. Also, adesired Ge concentration profile as a function of the Si_(1−x)Ge_(x)film thickness is input into processor 110. For example, Si_(1−x)Ge_(x)film 30 in FIG. 1 is formed for input germanium concentrations of 20% atpoint 40 and 0% at point 35 having a linear change in concentration frompoint 40 to point 35 over a 500 Angstroms Si_(1−x)Ge_(x) input filmthickness identified as film 30.

When wafer 10 is placed in processor 110, processor 110 first calculatesthe curves of FIGS. 4 and 5 using the six experimentally determinedmeasures for concentration and growth rate. Processor 110 next uses thedesired input concentration profile over the desired growth thickness asa guide to calculate the set points for GeH₄ mass flow controller 162.For a desired Ge concentration, the corresponding GeH₄ flow rate iscalculated from FIG. 4. This flow rate is then used to calculate theSi_(1−x)Ge_(x) growth rate, from FIG. 5. The corresponding growth rateused along with a selected time interval (Δt) establishes the desiredgrowth thickness of a portion of Si_(1−x)Ge_(x) film for the timeinterval. The thickness of the Si_(1−x)Ge_(x) grown within a selectedtime interval is subtracted from the total desired film thickness toestablish the thickness left to be grown. Using the new thickness thatyet needs to be grown, the desired input concentration profile as afunction of thickness is used to calculate a corresponding newSi_(1−x)Ge_(x) concentration value. Using the new concentration value,the above process of using the data from FIGS. 4 and 5 will be repeatedto calculate a new thickness of Si_(1−x)Ge_(x) grown for a second timeinterval, Δt. This iterative process will continue until the totaldesired thickness of Si_(1−x)Ge_(x) is on wafer 10.

In one example, a Si_(1−x)G_(x) film has a Ge concentration of 20percent at the wafer film interface (concentration point 40) and zero atthe film surface (concentration point 35). In this example, a linearconcentration profile is desired. Given the desired concentration (e.g.,20 percent), the data obtained from FIG. 4 is queried to obtain thedesired flow rate of GeH₄ species through mass flow controller 162 (flowas a function of concentration). Once the flow is established, the datacollected and represented by FIG. 5 is utilized to calculate a growthrate for the desired flow (growth rate as a function of flow rate). Fora predetermined time interval (e.g., 0.2 seconds), the amount of film 30introduced on substrate 25 during a time interval of 0.2 seconds may bedetermined for the desired concentration. Thus, by using theexperimental data and a predetermined time interval, the concentrationin a Si_(1−x)Ge_(x) film and a film thickness is known.

In the example where a linear variance in concentration is desired, suchas illustrated by line 200 in FIG. 2, a corresponding concentration isdetermined for a second time interval. Thus, in reference to FIG. 2,knowing the concentration profile as a function of film thickness andstarting from a desired concentration point of film 30, subsequentconcentration points along the path of line 200 may be calculated fortime intervals, Δt. In one embodiment, a time interval of 0.2 seconds isused to control mass flow controller 162 and the corresponding GeH₄ flowrate (FIG. 4) as well as growth rate (FIG. 5) is calculated to obtain alinear profile (line 200). The process continues until a desired filmthickness of film 30 is formed.

Processor 110 contains, in one embodiment, a suitable algorithm tocalculate the desired flow rate of a constituent to mass flow controller162 as a function of concentration. Processor 110 also contains, in thisembodiment, a suitable algorithm to calculate a growth rate as afunction of flow rate. For example, processor 110 is supplied withsoftware instruction logic that is a computer program stored in acomputer-readable medium such as memory in processor 110. The memory is,for example, a hard disk drive. Additional memory associated withprocessor 110 stores, among another items, the experimentally determineddata of concentration of the constituent species over a desired flowrate spectrum (FIG. 4), and experimentally-determined data related tothe growth rate of the constituent species over the desired flow ratespectrum (FIG. 5) as well as the corresponding curve fit algorithms.

FIG. 6 shows an illustrative block diagram of the hierarchical structureof system logic according to one embodiment of the invention for forminga film having a desired concentration profile and thickness on asubstrate which is a wafer. Such control logic would constitute aprogram to be run on processor 110. A suitable programming language forsuch a program includes, but is not limited to, C, C⁺⁺, and otherlanguages. The program may be supplied directly on processor 110 or toprocessor 110 by way of an outside device, such as a computer.

As a first operation, certain user inputs are supplied to processor 110and stored in the form of either internal or external memory. Theinformation supplied to processor 110 for the system logic includesexperimental data for introduction rate, e.g., concentration of aspecies such as Ge concentration as a function of mass flow rate (block310), experimental data for growth rate of an introduced film, such asSi_(1−x)Ge_(x), as a function of mass flow rate (block 320), the desiredthickness of a film on a substrate, such as a Si_(x)Ge_(1−x) film on awafer (block 330), and the desired concentration profile of a filmformed on a substrate such as a wafer (block 340).

Once the above-described data is supplied to processor 110, the systemlogic calculates a flow rate of a species, such as GeH₄, for a value ofconcentration desired by the user (block 350) for a predetermined timeinterval. System logic is then used to control mass flow controller 162to regulate the corresponding flow rate of source gas 130 of GeH₄.

In addition to calculating a corresponding flow rate of a species for adesired concentration, the system logic calculates an introduction rate,e.g., a growth rate, of a corresponding film on a substrate, such as awafer for the calculated flow rate (block 360). For a calculated growthrate of film, an amount of film can further be calculated for a giventime interval (block 370). This information is used by processor 110 tointroduce a constituent, such as GeH₄, through mass flow controller 162to introduce a film on substrate 25 for a selected time interval. 0.2seconds is an example of a desired time interval as 0.2 secondsrepresents the time interval utilized for ramp-up or ramp-down of flowthrough a mass flow controller, for example, a UNIT mass flow controllerused in an EPI Centura system.

Once a constituent is introduced into chamber 100 to form a portion offilm 30 according to the method described herein for a predeterminedtime interval, the system logic determines whether the desired filmthickness is achieved by comparing the calculated film thickness withthe desired film thickness (block 380). If the desired input filmthickness with the desired input concentration profile has not beenachieved, the system logic of processor 110 calculates a new value forfilm thickness representing the additional thickness amount needed toobtain the total desired input thickness (block 390), and calculates thecorresponding desired input concentration value for the newly calculatedthickness (block 395). Processor 110 returns to block 350 and uses thenewly calculated value of desired concentration to calculate acorresponding flow rate. Processor 110 continues this process until adesired film thickness is achieved. Once a desired film thickness isachieved, the system logic discontinues the loop and completes the filmformation (block 396). It is to be appreciated that calculations of flowrates and film growth may precede the introduction of a constituent intothe chamber.

According to the method and system described, a film-forming constituentcan be controlled, through control of a mass flow controller, to achievea desired concentration profile of a species in a film and a desiredfilm thickness introduced on a substrate, such as a wafer. In the aboveembodiment, a method of achieving a linear concentration profile isdescribed. FIG. 7 shows a SIMS profile of a epitaxially grownSi_(1−x)Ge_(x) film on a silicon substrate having a linear concentrationprofile of Ge introduced according to a method of the invention. TheSIMS profile illustrates the atomic profile of Ge from the surface (0depth) to the interface of the Si_(1−x)Ge_(x) and the silicon substrate.Thus, the depth represents the depth into the Si_(1−x)Ge_(x) film.

In FIG. 7, the thickness of the Si_(1−x)Ge_(x) film is approximately1000 Å. The concentration at the surface of the film is zero(represented as beginning at a depth of approximately 500 Å to accountfor a cap on the SIMS system). The concentration profile through thefilm is linear to a Ge concentration of 16 percent at theSi_(1−x)Ge_(x)/silicon interface.

FIG. 8 shows a plot of the flow rate of GeH₄ introduced to produce thelinear Ge profile illustrated in FIG. 7. FIG. 8 illustrates that alinear concentration profile is formed but the flow rate of GeH₄ thatproduced the profile is varied in a non-linear fashion.

It is to be appreciated that the principles of the invention are notlimited to a method and system for introducing a film having a linearconcentration profile of a species constituent, but are equallyapplicable to situations where a non-linear concentration profile isdesired. For example, a profile such as illustrated by lines 210 and 220in FIG. 2 or other profile may be desired. In one aspect, the inventionprovides a technique for controlling a mass flow controller to achieve adesired concentration profile in a film introduced on a substrate.

FIG. 9 shows a film profile of epitaxially grown silicon-germanium filmon a silicon substrate having a concave concentration profile of Geformed according to a method of the invention. Similar to FIG. 7, thefilm's profile illustrates the atomic profile of Ge from the surface(zero depth) to the interface of the Si_(1−x)Ge_(x) and the siliconsubstrate. The thickness of the silicon-germanium film is approximately1000 Å. The concentration profile adapts a concave representation from aGe concentration of zero percent at the surface of the film to aconcentration of 15 percent at the interface.

FIG. 10 shows a plot of the flowchart of GeH4 introduced to produce theprofile illustrated in FIG. 9. FIG. 10 illustrates that the concaveprofile produced in FIG. 9 is not the result of a linear change in theflow rate of GeH₄.

In addition to providing the ability to establish a desiredconcentration profile of a film introduced on a substrate, such as awafer, the invention offers a method and system for defining thethickness of a film introduced on a wafer. Accordingly, the inventionmay be practiced so as to achieve a desired concentration profile of aspecies, introduced by mass flow meter, having a desired concentrationprofile and a desired film thickness.

One application of controlling the introduction of constituents onto asemiconductor substrate to yield a graded film is in the formation ofheterojunction bipolar transistors (HBTs). Bipolar transistors areutilized in a variety of applications including as amplifying andswitching devices. HBTs generally offer improved performance overtraditional bipolar transistors and metal oxide semiconductor (MOS)transistors in high frequency applications, particularly applicationsapproaching 50 gigahertz (gHz). As higher frequency applications (e.g.,50 gHz or greater) become desirable, a need for improved HBTs exist. Theinvention contemplates improved performance of HBTs by utilizingSi_(1−x)Ge_(x) graded junctions having optimized concentration profiles.

FIG. 11 shows a representative example of an HBT according to theinvention. HBT 400 includes emitter region 410, base region 420 andcollector region 430. Emitter-base (E-B) spacer 435 is positionedbetween emitter region 410 and base region 420. Base-collection (B-C)spacer 445 is positioned between base region 420 and collector region430. HBT 400 is characterized by base region 420 of an epitaxiallyformed Si_(1−x)Ge_(x) film as are E-B spacer 435 and B-C spacer 445. Thebipolar transistor, in this embodiment, comprises N-type emitter region410, P-type base region 420, and N-type collector region 430 (a NPNtransistor). FIG. 11 illustrates the movement of electrons throughtransistor 400 in response to a voltage applied through electrode 450.In one example that follows, at least E-B spacer 435 will be formed withconcentration gradient analagous to that shown in FIG. 9 (i.e.,profile).

The use of Si_(1−x)Ge_(x) in an HBT generally enables high-frequencyperformance. One of the major advantages of Si_(1−x)Ge_(x) is a smallerenergy gap than that of silicon. In unstrained bulk Si_(1−x)Ge_(x), theenergy gap drops from approximately 1.1 electron-volts (eV) in siliconto 1.0 eV for Si_(0.8)Ge_(0.2). The lattice constant of Si_(1−x)Ge_(x)is also larger than the lattice constant in silicon. If the thickness ofthe Si_(1−x)Ge_(x) alloy is below a critical value, the mismatch inlattice constant is accommodated elastically, no dislocations areformed, and the Si_(1−x)Ge_(x) film is strained. Strain lifts thedegeneracy of both valence and conduction bands. As a result, the energygap of strained Si_(1−x)Ge_(x) decreases even more than unstrainedSi_(1−x)Ge_(x), to approximately 0.9 eV for strained Si_(0.8)Ge_(0.2).

The change in the energy gap in strained Si_(1−x)Ge_(x) film, Δ, allowsfor a fast transmit of charge carriers in the base region (e.g., baseregion 420) of HBTs under the action of the drift electric field, E:

E=−dΔ/dl,  (1)

where l is the distance across the base region. For example, the bandgap reduction of 0.2 eV across a 500 Å base region, translates to adrift electric field of 40 kV/cm.

Charge carrier drift mobility, μ, through the base region of an HBT isproportional to the scattering time, τ, and inversely proportional tothe effective mass of the carrier, m*:

μ˜τ/m*.  (2)

The scattering time diminishes with increasing Ge concentration becauseof alloy scattering. The effective mass, m*, becomes anisotropic between“in-plane” and “perpendicular to the junction” directions of motionbecause of strain. For the perpendicular direction, the effective massof holes is significantly smaller than Si_(1−x)Ge_(x) due to valenceband offset. The resulting hole mobility augments with the increase ofx, from 450 cm²/Vs for x=0 to 1000 cm²/Vs for x=0.2 in Si_(1−x)Ge_(x)with low dopant (i.e., P-type, N-type) concentration. The opposite trendtakes place for electrons: their perpendicular mobility drops from 14000cm²/Vs for x=0 to 750 cm²/Vs for x=0.2. Despite the decrease in theelectron mobility with increasing x, the electron mobility is largerthan the hole mobility over the majority of the practically useful rangeof the Ge concentration (i.e., x between 0 and 0.2). Accordingly, thetransistor configuration of choice for high-speed applications isgenerally the NPN transistor because the species travelling from theemitter region to the collector region are electrons.

Typical P-type doping levels for Si_(1−x)Ge_(x) base region 420 of NPNtransistor 400 are in the 10¹⁸ cm⁻³-10¹⁹ cm⁻³ range. In one aspect, thischoice is generally determined by the requirement of having fairly lowsheet (i.e., in-plane) resistance of base region 420. When the dopantconcentration becomes large, two effects are generally thought to occur:first, the carrier perpendicular mobility is significantly reduced. Forinstance, for electrons, the perpendicular mobility is approximately 250cm²/Vs and 120 cm²/Vs for doping levels of 10¹⁸ cm⁻³ and 10¹⁹ cm⁻³,respectively. Second, the mobility dependence on the Ge concentration isreduced: carrier mobility, μ, is almost x-independent for doping levelsin the 10¹⁸ cm⁻³-10¹⁹ cm⁻³ range. The effects of dopant concentration ispresented in detail in the treatise Semiconductors and Semimetals, Vol.56, “Germanium Silicon: Physics and Materials,” edited by R. Hull and J.C. Bean, in the article by S. A. Ringel and P. N. Grillot, “ElectronicProperties and Deep Levels in Ge—Si,” at pages 293-346 (1999). Thesignificant reduction in carrier mobility can be counterweighted byreducing the perpendicular size of the base (to 200Å-300 Å or thinner)and by the drift electric field (i.e., built-in potential) (see Equation1).

The small size of base region 420 generally results in degraded overallperformance of devices because of low leakage currents and reducedbreakdown voltages. In order to overcome this problem, it has beensuggested to use lightly doped spacers at the emitter-base andcollector-base regions, e.g., E-B spacer 435 and C-B spacer 445. SeeMeyerson, B. S., et al., “Silicon: Germanium Heterojunction BipolarTransistors; from Experiment to Technology,” Selected Topics inElectronics and Systems, Vol. 2, “Current Trends in HeterojunctionBipolar Transistors,” edited by M. F. Chang. E-B spacer 435 and C-Bspacer 445 may be intrinsic (i.e., no doping) or lightly N-type doped(e.g., typically below 10¹⁷ cm⁻³). As for lightly doped Si_(1−x)Ge_(x),the electron perpendicular mobility generally depends on the Geconcentration.

If the band energy gap changes linearly, the built-in electric field isconstant across the junction (Equation 1). This means that passingthrough the spacers, electrons spend significantly more time in the highGe concentration regions (where their mobility is lower) than they do inthe regions where the Ge concentration is lower. The inventionrecognizes that the overall transient time can be substantially reducedby creating a Ge concentration profile in such a way that the electricfield would be higher in the regions where mobility is lower. In otherwords, the electric field changes in an opposite way to that of themobility change (i.e., the electric field increases through theheterojunction if carrier mobility decreases and vice versa) to enhancethe cut-off frequency of the transistor.

In one example, the transient time, τ, through E-B spacer 435 isdetermined for the following two profiles of Ge concentration: Profile Ais a linear grade, and Profile B where the electric field is inverselyproportional to the electron perpendicular mobility to yield a concavegradient. As a simplification, a linear relationship between theelectron drift velocity, v, and the electric field is assumed:

v=μE.  (3)

In this example, E-B spacer 435 has a thickness, W, in which the Geconcentration changes from 0 to x₁. The corresponding changes in theband gap and in the mobility are from Δ₀=1.1 eV to Δ₁ and from μ₀=14000cm²/Vs to μ₁. The transient time is given by the integration across baseregion 420 from 0 to W:

τ=∫dl/v[x(l)].  (4)

Note that the velocity is a function of x which in turn is a function ofthe distance, l, across base region 420.

Literature data on the band gap and mobility for x=0-0.2 can be closelyapproximated by the following linear relations:

Δ=Δ₀ −αx, μ=μ ₀ −βx,  (5)

respectively, with α=1 eV and β=3250 cm²/Vs. For Profile A, the electricfield is constant: E=αx1/W, mobility is μ=μ₀−(βx₁/W)1, and Equation 4 isreduced to

 τ=(W/a x ₁)∫dl/(μ₀ −β*l),  (6)

with β*=βx₁/W. Integrating Equation 6, the transient time of electroncarriers through E-B spacer 435 for Profile A becomes:

τ_(A)=(W ² /αβx ₁ ²)ln(μ₀/μ₁).  (7)

For Profile B, the velocity is constant: v=v_(0,) and therefore

τ_(B) =W/ v ₀.  (8)

From Eqs. 1 and 5, we have for the electric field:

E=αdx/dl.  (9)

Using Eqs. 3, 5 and 9, we obtain:

αdx/dl(μ₀−β_(x))=v₀.  (10)

After integration Equation 9 and substitution for the transient time ofelectron carriers through E-B spacer 435 in Profile B becomes:

τ_(B) =W ²/(αμ₀ x ₁ −αβx ₁ ²/2).  (11)

From Eqs. 9 and 11, we finally have:

τ_(A)/τ_(B)=[(μ₀ /β−x ₁ ²/2)ln(μ₀/μ₁)]/x ₁  (12 )

Substituting data for x₁=0.15, Equation 12 becomes: τ_(A)/τ_(B)=1.5.

According to the above analysis, the transient time for a concentrationprofile according to Profile B is shorter than for the linear profile ofProfile A. For Profile B, the electric field is smaller near emitterregion 410 and increases towards base region 420. This change inelectric field may be attributed to the band gap changing more slowlynear emitter region 410 than it does near base region 420 (see Equation1). According to Equation 5, this in turn means that for Profile B, theGe concentration changes more slowly near emitter region 410 and fasternear base region 420. In other words, moving across E-B spacer 435, theGe profile has a concave curvature (similar to line 220 in FIG. 2) withthe Ge concentration being smaller on the emitter side of E-B spacer 435and increasing to a maximum at the interface between E-B spacer 435 andthe base region.

In the above example, a NPN HBT transistor was described having aSi_(1−x)Ge_(x) base region with an E-B spacer and a B-C spacer. Thespecific example described the E-B spacer. It is to be appreciated thatsimilar beneficial results may be obtained with a similarly optimizedB-C spacer, as well as an optimized base region. It is also to beappreciated that hole mobility generally increases with increasing Geconcentration (a behavior opposite to that of the electron mobility).Therefore, in the case of a PNP HBT, a convex profile (similar to line210 of FIG. 2) of Ge concentration in a spacer will generally result inthe shortest transient time and the highest cut-off frequency of PNPHBTs.

In the preceding detailed description, the invention is described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention as setforth in the claims. The specification and drawings are, accordingly, tobe regarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. A method comprising: controlling the flow rate ofa species according to a determined concentration profile of a filmcomprising the species introduced on a substrate; and introducing a filmon a substrate, the film comprising the species at a first concentrationat a first point in the film and a second concentration different thanthe first concentration at a second point in the film.
 2. The method ofclaim 1, wherein determining the concentration profile comprises:determining a concentration of the species introduced on a substrate fora first plurality of flow rates; determining a growth rate of thespecies grown on a substrate for a second plurality of flow rates; anddetermining a concentration profile of the species for a unit of time.3. The method of claim 1, wherein the introduced film comprises athickness, the method further comprising: controlling the flow rate tointroduce the film at a graded concentration of the species throughoutthe thickness of the film.
 4. The method of claim 3, wherein the flowrate is controlled so that the graded concentration of the speciescomprises a linear gradient.
 5. The method of claim 1, whereincontrolling the flow rate comprises controlling the mass flow rate ofthe species.
 6. The method of claim 1, wherein the introduction of thefilm on a substrate comprises introducing the species and growing thefilm on the substrate.
 7. A method comprising: forming a film on asubstrate, the film comprising a film thickness defined between twopoints and a concentration profile of a species, the concentrationprofile varying between the two points according to a determinedrelationship between the species flow rate and a concentration of thespecies within a film.
 8. The method of claim 7, wherein theconcentration profile varies linearly.
 9. The method of claim 7, whereinforming the film comprises introducing the species and growing the filmon the substrate.
 10. A method comprising generating a concentrationprofile specifying a concentration of a species in a film formed on asubstrate by relating a species concentration in a film to a speciesflow rate during formation of the film.
 11. The method of claim 10,wherein the concentration profile specifies a concentration in a filmgrown on the substrate, and generating the concentration profile furthercomprises relating a film growth rate to a species flow rate.
 12. Themethod of claim 10, further comprising: forming a film on a substratecomprising a varying concentration of the species across a thickness ofthe film according to the generated concentration profile.